If you install XP on the second hard drive it will rewrite the boot manager
and Vista will not start.
Vista uses a different and incompatible boot manager than XP.
Unfortunately the automated tools on the Vista install disc are not very
effective at reinstalling the Vista boot manager which is capable of
recognizing both the vista and XP installs IF you installed Vista after XP.
Having recently run into that issue I was pleasantly surprised to find that
the automated tools on the Win7 install disc very effectively restored the
Vista boot manager, recognized and configured both the Vista and XP installs
without having to install Win7, when the Vista install disc tools failed to
do so after multiple tries. You can then control which will be the default
OS and how long the choice menu will display through Vista.